In order to obtain maximum function and efficiency in a semiconductor package having minimum dimensions, various approaches have been developed for increasing semiconductor device density in packages. Among the various types of packages are the so-called “multi-chip module”, “multi chip package” or “chip stacked package”. Some types of multi-chip modules include assemblies of semiconductor devices that are stacked one on top of the other. Stacking semiconductor devices like this can save a large amount of the space in the package.
Wafer dicing is carried out before the chip bonding process. Conventionally, the back of the wafer is coated with a polymer film that provides adhesion between adjacent stacked semiconductor chips or between a chip and a substrate. For stacking semiconductor chips, each chip is lifted by a chip bonding tool, which is typically mounted at the end of a pick-and-place device, and carried to an mounted onto a chip or a substrate under conditions of temperature in a range about 25° C. to about 180° C., and forces in a range about 0.5 N to about 10 N, depending on the chip size and thickness, among other parameters. Such a mounting process is referred to as “film bonding” or as a “pre-coated chip process”.
Film bonding has a number of advantageous features, particularly as compared with techniques using a curable adhesive resin for die attach. For example, the thickness of adhesive film is uniform and the adhesive film as applied onto the wafer and subsequently sawn provides an accurately dimensioned adhesive coating over the entire back of the semiconductor chip. Consequently, there is no die tilt, and adhesive voids and adhesive fillet encircling the semiconductor chip are avoided. Moreover, there is no resin bleed, making the process particularly suitable for die stacking and packages with tight design tolerances.
Semiconductor chips are being made increasingly thin in order to decrease the package size. Semiconductor chips having thicknesses approximately 150 μm and less can be bent when a conventional pneumatic (suction) chip bonding tool is used to pick-and-place the chip during the chip bonding process. In a film bonding process, when the semiconductor chip is picked by the chip bonding tool, the suction causes the chip to bend owing to the difference between the ambient (atmosphere) pressure and the pressure within the tool created by the vacuum. On placing the semiconductor chip onto a substrate or onto a lower adjacent chip in a chip stack, undesired air bubbles form between the chip being placed and the underlying chip or substrate surface because the bent chip first comes to rest with its edges on the underlying surface. The void formed between the downward facing surface of the chip and the underlying surface of can become sealed so that the trapped air cannot escape. Air trapped in this manner at the adhesion line between die and substrate or between adjacent stacked die degrades the reliability and performance of the package.